Method for restoring local polymer coating of a previously stripped optical fibre

ABSTRACT

The invention concerns a method for restoring the coating of a previously stripped optical fibre, characterised in that it comprises steps which consist in: applying a drop of viscous substance on one end of the fibre ( 10 ) stripped zone, at the interface ( 22 ) with the remaining initial coating ( 20 ), and shaping said drop into a mass ( 30 ) centre on the fibre ( 10 ) axis, tapering away from the adjacent initial coating ( 20 ), before filling up the fibre stripped space with a mass of substance capable of sheathing said fibre ( 10 ) again.

[0001] The present invention relates to the field of optical fibers.

[0002] More specifically, the present invention relates to thereconstitution of the protective coating, generally polymer-based, of anoptical fiber prestripped over a short spatial dimension.

[0003] During the production of various components, for examplefrequency filters, integrated into an optical fiber, it is very oftennecessary to remove the protective coating of the fiber (see FIG. 1).This coating is usually a polymer, such as an acrylate, the crosslinkingof which is obtained by exposure to UV radiation. To allow and/orfacilitate the manufacture of the component, the polymer is thereforeremoved from the fiber by means of various mechanical, thermal orchemical methods [1]. After the component has been produced, the polymersheath is reconstituted in order to improve the mechanical behaviour ofthe fiber and to prevent any contamination by oxidizing agents.

[0004] There are in the market several companies supplying machinescapable of reconstituting the sheathing of the fiber [2]. Their maincustomers are, at the present time, optical telecommunication systemsand equipment manufacturers working on terrestrial applications. Asregards the submarine telecommunications market, this is much moredraconian in terms of mechanical strength and lifetime of thecomponents. However, very few machines are capable at the present timeof meeting most of the conformity criteria regarding fiber resheathingquality. The main shortcoming of these machines is the delamination (ordebonding) which occurs between the sheathed original region and thereconstituted region (see FIG. 2). The presence of air bubbles 40 (seeFIG. 1) at this interface reduces the mechanical strength of the opticalfiber over time.

[0005] It is now an object of the present invention to provide novelmeans making it possible to improve the local reconstitution of thecoating, especially polymer coating, of a prestripped optical fiber overthe known prior techniques.

[0006] This object is achieved within the context of the presentinvention by virtue of a method of reconstituting the coating of aprestripped optical fiber, characterized in that it comprises the stepsconsisting in:

[0007] applying a drop of a viscous material, for example of polymer orsilicone, on one end of the stripped region of the fiber, at theinterface with the remaining initial coating, and

[0008] shaping this drop into a mass which is centered on the axis ofthe fiber and tapered on going away from the adjacent initial coating,before

[0009] filling the stripped space of the fiber with a mass of materialcapable of resheathing said fiber.

[0010] The present invention also relates to the fibers obtained afterimplementing this reconstitution method.

[0011] According to another advantageous characteristic of the presentinvention, the shaping step consists in shaping the drop of viscousmaterial into a mass having a generally frustoconical envelope.

[0012] Other features, objects and advantages of the present inventionwill become apparent on reading the detailed description which follows,together with the appended drawings, given by way of nonlimitingexamples and in which:

[0013]FIG. 1 shows a known silica-based virgin optical fiber, the coreof which having a diameter of 125 microns is covered with an acrylatepolymer sheath having a diameter of 250 microns;

[0014]FIG. 2 shows the resheathing obtained with a machine commerciallyavailable at the present time, in which figure bubbles trapped at thesheathed virgin region/resheathed region interface may be seen;

[0015]FIG. 3 show a fiber stripped, within the context of the presentinvention, so as to obtain a sharp and clean interface;

[0016]FIG. 4 shows the stripped fiber provided with a cone created atthe interface with the original sheathing with the aid of a drop ofmolded polymer, in accordance with the present invention, the cone thenbeing cured by UV radiation or by raising the temperature;

[0017]FIG. 5 shows an enlarged view of this cone, revealing a gentleslope at the interface between the stripped region and the nonstrippedregion, the length of the cone illustrated in FIG. 5 being between 1 and5 mm; and

[0018]FIG. 6 shows the final optical fiber after stripping, manufactureof the cones in accordance with the present invention and resheathing.

[0019] It will be noted that, in FIG. 6, no trapped air bubble ordebonding at the interface may be seen, unlike what may be seen in aconventional fiber as illustrated in FIG. 2.

[0020] The method of reconstituting the polymer coating of a prestrippedoptical fiber, in accordance with the present invention, will now bedescribed in more detail with regard to the appended FIGS. 3 to 6.

[0021] Firstly, within the context of the invention, the fiber 10 isstripped of its polymer coating 20 over a region of a fewmillimeters/centimeters with a sharp cut into the polymer, asillustrated in FIG. 3, that is to say with a clear-cut interface 22transverse to the axis of the optical fiber.

[0022] This makes it possible to prevent bubbles close to the sheathedfiber/air interface 22 from being trapped.

[0023] After the component has been produced in the fiber 10, a drop ofpolymer (of the order of a few mm³ and of viscosity equal to 5000 mpa.s)is deposited at each end of the stripped region. This drop is applied,as illustrated in FIGS. 4 and 5, between the silica fiber 10 and thepolymer interface 22. It is then molded in order to assume the shape ofa cone 30 centered on the axis of the fiber, with a diameter at its baseof around 250 to 350 microns (for a fiber with an initial coating of 250μm).

[0024] The shaping of the drop may be performed manually or with amachine designed for this purpose.

[0025] The polymer must be viscous enough to facilitate this operation.By way of indication, the viscosity must preferably be between 1000 and10 000 mPa.s. Next, the two cones are crosslinked/cured by subjectingthem for a few seconds/minutes to ultraviolet radiation or to a rise intemperature by any other suitable means. Furthermore, if the polymer isviscous enough, this step of crosslinking the cones is not necessary.

[0026] This operation may be repeated several times until the desiredshape and structure are obtained. That is to say, in order to obtain thedesired final cone, it is possible to deposit in succession severaldrops of polymer with successive shaping of each of them.

[0027] Once the cones have been formed, any commercially availablemachine for resheathing the central part of the stripped region can beemployed. This machine delivers a given amount of polymer (depending onthe length) which is distributed around the fiber, to be subsequentlycrosslinked.

[0028] Such resheathing may be carried out conventionally in one or moresuperposed layers of polymers over the entire stripped length.

[0029] Because of the presence of the cones 30, and therefore of agentle interface between the sheathed and unsheathed regions, the finalstep of the reconstitution takes place without the appearance of airbubbles or without delamination.

[0030] Preferably, within the context of the invention, the inventorshave found that the apex angle of the cone 30 must be between 5° and 70°in order to obtain an optimal result.

[0031] The resheathed final component with the cones according to thepresent invention is illustrated in FIG. 6.

[0032] Of course, the present invention is not limited to the particularembodiment which has just been described, but extends to any variant inaccordance with its spirit.

[0033] In particular, the invention is not limited to the strict use ofpolymers for producing the cones. The present invention may beimplemented with the aid of any equivalent material, such as with asilicone material for example.

[0034] Preferably, as indicated above, the material used to produce thecones is, however, thermally or UV-radiation crosslinkable. Of course,this facilitates the growth of the cone by successively depositingseveral drops and/or the complete resheathing of the prestripped region.Furthermore, the process can also be used for a fiber having a 400 or900 μm coating.

[0035] The present invention applies particularly to the resheathing ofoptical fibers comprising integrated optical functions. This is becausethe present invention makes it possible to strip and resheath suchfibers without impairing their mechanical strength over time. Inparticular, the present invention allows the fibers to be completelyprotected from external perturbations without modifying their mechanicalbehavior. The present invention applies in general to any optical fiber(filter, splice, etc.) requiring the removal then the local resheathingof the fiber.

[0036] The present invention is especially applicable in the submarinetelecommunications market and sensor devices which require longcomponent lifetimes.

[0037] The invention applies most particularly, but not exclusively, tothe resheathing of an optical fiber in which a Bragg grating has beenphotowritten. At the present time, such a component is a key element oftelecommunications and makes it possible in particular to carry outfunctions of filtering, isolating, stabilizing, extracting and routing alight wave [3].

[0038] Moreover, within the context of the present invention, thegeometry given, after shaping, to the mass coming from the drop ofviscous material deposited at the interface 22 with the remaininginitial coating may not be completely frustoconical, the essential pointbeing that this mass is tapered on going away from the said interface22, in order to join up with the outer surface of the fiber withvirtually no discontinuity (thereby corresponding to the expression“generally frustoconical envelope” used above).

[0039] [1] D. Varelas, “Mechanical reliability of optical fiber Bragggratings”, Doctoral thesis from the University of Lausanne(Switzerland), 1998.

[0040] [2] Technical documentation on Vytran Corporation resheathingmachines, 1999.

[0041] S. Boj, “Réalisations de filtres sélectifs en fréquence intégrésdans les fibres optiques et applications [Production offrequency-selective filters integrated into optical fibers andapplications]”, Doctoral thesis from the University of Lille, 1995.

1. Method of reconstituting the coating of a prestripped optical fiber, characterized in that it comprises the steps consisting in: applying a drop of a viscous material, on one end of the stripped region of the fiber (10), at the interface (22) with the remaining initial coating (20), and shaping this drop into a mass (30) which is centered on the axis of the fiber (10) and tapered on going away from the adjacent initial coating (20), before filling the stripped space of the fiber with a mass of material capable of resheathing said fiber.
 2. Method according to claim 1, characterized in that the shaping step consists in shaping the drop of viscous material into a mass (30) having a generally frustoconical envelope.
 3. Method according to one of claims 1 or 2, characterized in that the aforementioned steps of applying drops of viscous material and of shaping them are carried out on each end of the stripped region of the fiber.
 4. Method according to one of claims 1 to 3, characterized in that it consists in repeating several times the steps of applying a drop of viscous material and of shaping it before the filling step is carried out.
 5. Method according to one of claims 1 to 4, characterized in that the viscous material is a polymer.
 6. Method according to one of claims 1 to 4, characterized in that the viscous material is a silicone.
 7. Method according to one of claims 1 to 6, characterized in that it furthermore includes the step consisting in crosslinking the viscous material before the shaping step.
 8. Method according to one of claims 1 to 7, characterized in that it furthermore includes the prior step consisting in making a sharp cut in the initial coating of the fiber, preferably in a plane orthogonal to the axis of the fiber (10).
 9. Method according to one of claims 1 to 8, characterized in that the stripped region of the fiber (10) has a length of between a few millimeters and a few centimeters.
 10. Method according to one of claims 1 to 9, characterized in that the volume of each drop of viscous material deposited at each application step is of the order of a few mm³.
 11. Method according to one of claims 1 to 10, characterized in that the diameter at the base of the cone (30) is around 250 to 350 microns.
 12. Method according to one of claims 1 to 11, characterized in that the apex angle of the cone (30) is around 5 to 70°.
 13. Method according to one of claims 1 to 12, characterized in that the viscosity of the material applied is between 1000 and 10 000 mPa.s.
 14. Method according to one of claims 1 to 13, characterized in that it furthermore includes the step consisting in forming a Bragg grating in the stripped region of the fiber (10) before it is resheathed.
 15. Optical fiber obtained by implementing the method according to one of claims 1 to
 14. 16. Fiber according to claim 15, characterized in that it comprises two cones (30) respectively adjacent to the end interfaces of a locally removed original coating (20), these being covered with a final resheathing. 